Radiolabeled metabolites of proteins play a critical role in radioactivity elimination from the liver

Abstract

We have recently reported that the behavior of radiolabeled metabolites in the liver appears to be responsible for the hepatic radioactivity levels after administration of protein radiopharmaceuticals. To better understand the role played by radiolabeled metabolites in hepatic radioactivity levels, two benzyl-EDTA derivatives rendering different radiolabeled metabolites, 1-(4-isothiocyanatobenzyl) ethylenediaminetetraacetic acid (SCN-Bz-EDTA) and 1-[ p-(5-maleimidopentyl)aminobenzyl] ethylenediaminetetraacetic acid (ECMS-Bz-EDTA), were selected as bifunctional chelating agents (BCAs), and 111In labeling of galactosyl-neoglycoalbumin (NGA) and mannosyl-neoglycoalbumin (NMA) was performed. Biodistribution of radioactivity in mice and subcellular distribution of radioactivity in hepatocytes were then compared. After accumulation in hepatic parenchymal cells, NGA-EMCS-Bz-EDTA- 111 In rendered a faster elimination rate of radioactivity from the liver than NGA-SCN-Bz-EDTA- 111 In. Although each 111In-NMA exhibited a delayed elimination rate of radioactivity from the liver compared to the 111In-NGA counterpart, NMA-EMCS-Bz-EDTA- 111In showed faster elimination rate of radioactivity than NMA-SCN-Bz-EDTA- 111In. Analyses of radioactivity excreted in feces and urine and remaining in the liver indicated that both BCAs rendered mono-amino acid adducts as the major radiolabeled metabolites (cysteine-EMCS-Bz-EDTA- 111In and lysine-SCN-Bz-EDTA- 111In), which were generated in both cell types of the liver within 1 h postinjection. Subcellular distribution of radioactivity indicated that the radioactivity was copurified with lysosomes. These results demonstrate that although in vivo stability of radiometal chelates is essential, the biological properties of the radiolabeled metabolites generated after lysosomal proteolysis in hepatocytes play a critical role in radioactivity elimination from the liver.